The present invention relates to a power divider/combiner applicable to communication equipment, measurement equipment, and some other equipment with a high-frequency circuit incorporated therein. More particularly, this invention relates to a power divider/combiner used mainly in a band from a quasi-millimeter wave to a millimeter wave and a submillimeter wave.
As an example of a conventional type of power divider/combiner, there is the one shown in FIG. 6. In this figure, designated at the reference numerals 1, 2, and 3 are I/O terminals, at 4 a branch section, at r an absorption resistor, and at b a quarter wavelength line.
Two quarter wavelength lines b are connected to the I/O terminal 1 via the branch section 4, and the other ends of the quarter wavelength lines are connected to the I/O terminals 2 and 3 respectively. The I/O terminal 2 and the I/O terminal 3 are connected to each other through the absorption resistor r.
Operations of this apparatus are explained below. A signal supplied from the I/O terminal 1 is branched into the two quarter wavelength lines b with a uniform amplitude, and the branched signals are fetched from the I/O terminal 2 and the I/O terminal 3. Each of the quarter wavelength lines b operates as an impedance converter and matches a characteristic impedance Z0 of the I/O terminal 1 to that of each external circuit connected to the I/O terminal 2 and I/O terminal 3. The absorption resistor r absorbs unbalanced components of the I/O terminals 2 and 3 to provide isolation between the I/O terminal 2 and the I/O terminal 3.
FIG. 7A and FIG. 7B show a S parameter when a design frequency in the power divider/combiner shown in FIG. 6 is set to 25 GHz. FIG. 7A is shown in dB, and FIG. 7B shows a Smith chart. As shown in this figure, the amount of reflection S11 and S22 from each of the I/O terminals and the isolation S23 between the I/O terminal 2 and the I/O terminal 3 are zero at the design frequency of 25 GHz, which shows that a complete matching and isolation is achieved therebetween. In this figure, lines S11 and S23 are seen as one line, but in fact are there are two separate lines that are superimposed on each other each representing S11 and S23. The above explanation is for a case where power is distributed using this power divider/combiner. However, when power is to be synthesized, a flow of signal is only in the opposite direction because a plurality of inputs are synthesized into one output. Accordingly, only the I/O terminals are replaced with each other and the other components in the circuit configuration are the same as those in FIG. 6, in which the relation between impedances or the like holds as it is. Therefore, only a case of power distribution is described below, and description of a case of power synthesis will be omitted.
Conventionally, the frequency used in a power divider/combiner was not so high. Therefore, the main technical object was how to minimize the size of the overall circuit. In recent years, however, the operational frequency of high frequency circuitry has been shifted from a microwave band to a millimeter wave band or a submillimeter wave band due to exhaustion of frequency resources as well as due to enhancement in performance of active elements in a semiconductor. In association with this tendency, the length of the quarter wavelength line became as short as around 1 mm or less. Therefore, presently the problem of size of the power divider/combiner itself is not as big as it used to be earlier.
Further, an absorption resistor has been considered as a lumped constant element in principle, so that a physical size of the resistor was not considered important. However, to assume that the absorption resistor is a lumped constant element, the size of the absorption resistor has to be made smaller according to miniaturization of the power divider/combiner. However, when the size of the absorption resistor is made smaller, the space between lines of the I/O terminals linked to each other via the absorption resistor becomes narrow, which causes design options to be restricted and increases the crosstalk.
On the other hand, in order to suppress crosstalk, for example, the power divider/combiner in FIG. 8 can be used. FIG. 8 shows a configuration of the power divider/combiner which suppresses crosstalk by providing lines c each between the absorption resistor r and each of quarter wavelength lines b. In this figure, the reference numerals 1 to 3 indicate I/O terminals, and the reference numeral 4 indicates a branch section.
FIG. 9 shows S Parameters when the line c whose electrical length is 20 degrees assuming that the design frequency in the power divider/combiner in FIG. 8 is 25 GHz. FIG. 9A is shown in dB and FIG. 9B shows a Smith chart. In this figure, lines S11 and S23 are seen as one line, but in fact there are two separate lines that are superimposed on each other each representing S11 and S23.
As shown in this figure, the isolation S23 between the I/O terminal 2 and the I/O terminal 3 is as high as xe2x88x9218.2 dB at the design frequency of 25 GHz. As described above, the power divider/combiner in FIG. 8 can suppress crosstalk, but the isolation between the I/O terminal 2 and the I/O terminal 3 worsens significantly.
As a basic transmission line especially for a high-frequency circuit such as an MMIC, an easily-designable micro-strip line has mainly been used. However, in recent years, the mainstream has shifted to a CPW which can be easily connected to a semiconductor device. Although the CPW has the characteristic of easy connectability to the semiconductor device because a signal line and a grounded conductor are located on one plane, its layout is complicated. Namely, an air bridge is required for a discontinuous section, and flexibility in the layout is significantly reduced as compared to that of the micro-strip line when a space between the lines described above is made extremely narrow.
As described above, in the Wilkinson type of power divider/combiner based on the conventional technology, a size of an absorption resistor has to be made smaller so that it is more negligible as compared to a wavelength of a design frequency. Therefore, there is a problem that flexibility in the layout or isolation between I/O terminals connected to each other via the absorption resistor is reduced. The problem described above becomes more obvious especially in the CPW which is popular in recent years.
It is an object of the present invention to provide a Wilkinson type of power divider/combiner with a high degree of flexibility in its layout and a high degree of isolation between I/O terminals connected to each other via an absorption resistor.
In the present invention, a transmission line with a length L having the characteristic impedance of {square root over (2)}xc2x7Z0 is provided between an absorption resistor and each of second I/O terminals and further between each of quarter wavelength lines and the second I/O terminal corresponding to the quarter wavelength line. Therefore, it is possible to provide a sufficient space between the I/O terminals with isolation therebetween via the absorption resistor kept at high level. As a result, a power divider/combiner in which crosstalk does not occur and the degree of flexibility in its layout is high can be obtained.
Further, the length L of the transmission line is set to a half wavelength or an integral multiple of the half wavelength. Accordingly, a characteristic impedance of this newly added transmission line is equivalent to that of a quarter wavelength line. Therefore, matching among all of the components is completely achieved so that the possibility of occurrence of unnecessary reflection is eliminated. Although complete matching is performed based on a design wavelength, there is the tendency that the frequency band width becomes narrower as the length of the connected transmission line increases. To overcome this problem, it is preferable that the transmission line to be connected is a half wavelength. As a result, a power divider/combiner in which matching of the all the components can be achieved, crosstalk does not occur and the degree of flexibility in its layout is high can be obtained.
Further, the circuit is formed with a CPW. Therefore, the space required for provision of an air bridge or the like, in other words, a space between I/O terminals connected to each other via an absorption resistor can be insured. As a result, a power divider/combiner in which crosstalk does not occur, the degree of flexibility in its layout is high and that can easily be connected to a semiconductor device can be obtained.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.